Walls, columns and other structures constructed of materials such as concrete or cement paste, brick or masonry units, and the like are widely used as support structures. Tunnels, building structural supports, bridge supports, freeway overpass supports and parking structure supports are just a few of the many uses for these cementitious structural supports. These supports may exist in a wide variety of shapes with circular, square and rectangular cross-sections being the most common. However, numerous other cross-sectional shapes have been used, including regular polygonal shapes and irregular cross-sections. The size of structural supports also varies greatly depending upon the intended use. Structural supports having heights and lengths exceeding 50 feet are commonly used in various applications.
It is common practice to reinforce concrete structural supports with steel rods, mesh, or bars. The steel reinforcement provides a great deal of added structural strength (e.g., compression, tensile, flexural and/or shear-resistance) to the support, but there have been numerous incidents of structural failure of these supports when subjected to asymmetric loads and horizontal displacement generated during earthquakes or explosions. Concrete structures, while adequate in compression, are subject to cracking, collapse, and partial loss due to stresses associated with earthquakes, explosions, land subsidence and overloading. Structural failure of such structures can have devastating consequences. Accordingly, there is a continuing need to enhance the ability of reinforced and unreinforced concrete and cement structural supports to withstand the asymmetric loads and horizontal displacements which are applied during an earthquake or explosion.
One way of increasing the structural integrity of support structures is to include additional metal reinforcement prior to forming the structural support. Other design features may be incorporated into the support structure fabrication in order to increase its resistance to asymmetric loading or horizontal displacement. However, there are hundreds of thousands of existing structural supports located in earthquake prone areas, which do not have adequate metal reinforcement or structural design to withstand high degrees of asymmetric loading or horizontal displacement. Accordingly, there is a need to provide a simple, efficient and relatively inexpensive system for reinforcing such existing structural supports to prevent or reduce the likelihood of failure during an earthquake or explosion.
One approach to reinforcing cementitious structures, such as concrete columns, is to wrap the exterior surface of the structure with a composite reinforcement layer, or fabric reinforced plastic (FRP). In U.S. Pat. No. 5,607,527 to Isley, Jr., a composite reinforcement layer having at least one fabric layer located within a resin matrix is wrapped around the exterior surface of a concrete column. The fabric layer has first and second parallel selvedges that extend around the circumferential outer surface of the column in a direction substantially perpendicular to the column axis. Preferred fibers disclosed by Isley include ones made from glass, polyaramid, graphite, silica, quartz, carbon, ceramic and polyethylene. Suitable resins suggested by this patent include polyester, epoxy, polyamide, bismaleimide, vinylester, urethanes, and polyurea, with epoxy-based resins being preferred.
Another approach to reinforcing a cementitious structural support is disclosed in U.S. Pat. No. 6,017,588 to Watanabe, et al. This patent discloses using an FRP to reinforce a structural support by forming a primer layer on the surface of the support structure, forming, if necessary, a putty layer on the primer layer, applying an impregnating resin on the primer layer (or putty layer) before, after or, before and after, cladding with fiber sheets to allow the resin to penetrate into the spaces in the fiber sheets, followed by curing the resin, the primer, putty and impregnating resin. The primer, putty and impregnating resin of this reference all include a resin composition. The disclosed fiber sheets may include carbon, aramid or glass fibers. The asserted advantage of the reinforcement structure of this patent is increased adherence of the reinforcement to the surface of the structural support.
Reinforcing FRP systems such as those described above can often be flammable, toxic and difficult to handle during application. They also provide, after curing, poor fire resistance, poor bonding to the concrete or brick being reinforced, and poor water/air permeability, resulting in the creation of moisture accumulation. Additionally, they are fairly expensive and tend to delaminate upon failure.
A repair or reinforcement system for existing support structures or for new construction support structures is needed.